1. Field of the Invention
The present invention relates to an organic light emitting display (OLED) device, and in particular relates to an active matrix organic light-emitting diode (AM-OLED) display device having a top cathode contact and image display system thereof.
2. Description of the Related Art
Flat display devices, such as organic light emitting display (OLED) and liquid crystal display (LCD), have many advantageous features, such as small dimensions, light weight, and low power consumption. Thus, flat display devices have been widely applied to electronic products, such as portable computers and mobile phones.
An organic light emitting display device having a self-emission feature adapts to applications for different environmental conditions, such as indoor applications and outdoor applications. Substantially, a conventional OLED device comprises an anode, an organic electroluminescent layer, and a cathode. A bias voltage may be applied between the anode and the cathode to activate the organic electroluminescent layer to emit light for image display. The technical trend of the OLED device is to improve light emitting efficiency and operating lifespan.
An active matrix organic light emitting display (AM-OLED) device comprises a matrix of active elements, such as an array of thin film transistors (TFT) and corresponding OLED pixels. An anode of each of the OLED pixels is controlled by the corresponding TFT. A cathode of the display device electrically connects to a control circuit in a peripheral region of the display device. A cathode of a conventional AM-OLED device forms an electrical connection from a lower substrate, such as an active matrix substrate, to other elements through a cathode contact. However, the contact area between the cathode and the cathode contact breaks easily due to bad step coverage of the cathode, which results in breakage or collapse of the cathode and further affects the quality and reliability of the device.
FIG. 1 shows a cross-section of a structure of a conventional AM-OLED device. Referring to FIG. 1, a conventional AM-OLED device 100 comprises a substrate 11 including an active region AR and a peripheral region PR. An array composed of a plurality of active elements 20 is disposed on the substrate 11 in the active region AR. The active element 20 may be, for example, a thin film transistor (TFT) comprising a gate 25, a gate dielectric layer 23, and a semiconductor layer 21 for forming a source/drain region and a channel region of the TFT. Passivation layers 12 and 13 are disposed on the substrate 11 to isolate each TFT and electrical connection. Signal lines 26A and 26B connect to the source region and the drain region of the TFT, respectively. The signal line 26B further connects to an anode 14 of each of the OLED pixels. An interlayer dielectric layer 13A is disposed on the passivation layer 13 to isolate each TFT and OLED pixel.
Pixel isolation structures 16 are disposed on the interlayer dielectric 13A to define and isolate each OLED pixel. Each OLED pixel comprises an anode 14 electrically connected to the signal line 26B through an electrical contact, organic electroluminescent layer 18 with different colors on the anode 14, and a continuative cathode 32 on the organic electroluminescent layer 18. A passivation layer 34 is disposed on the cathode 32.
Meanwhile, the continuative cathode 32 extends to the peripheral region PR of the substrate 11 and electrically connects to a signal line 28, such as a PVEE signal line, in a cathode contact opening. However, the step coverage of the cathode 32 near the step position 30 of the cathode contact opening is deficient, which results in breakage or collapse of the cathode and further affects the quality and reliability of the device.
In addition, U.S. Pat. No. 7,173,373 disclosed an AM-OLED device, wherein a plurality of cathode contacts are formed in the active element substrate to improve the electrical connection between the control circuit and the cathode. However, forming extra cathode contacts increases the complexity of the manufacturing process. For example, during patterning of the pixel electrodes and the contact holes, a more precise alignment system for photolithography is needed. Meanwhile, extra cathode contacts will also reduce the aperture ratio of the AM-OLED device.
Thus, it is desirable to have an AM-OLED display device and an image display system having a top cathode contact to prevent breakage or collapse near the contact position of the cathode.